A liquid crystal device (LCD) or liquid crystal on semiconductor (LCOS) device may be used in various applications, such as for example displays like in laptop computers, watches and calculators, and such as for example projection systems for projecting information or images onto a distant screen. An LCD or LCOS projector basically comprises a light generator such as a lamp for generating illumination light, illumination optics for capturing that light and transferring it to one or a plurality of LCD or LCOS devices comprising light valves, and a projection lens which images the illuminated LCD or LCOS device(s) on a screen. A basic property of both LCD and LCOS devices is that they work with polarised light, more particularly with linearly polarised light. Polarised light is used together with the properties of the liquid crystal elements to selectively on the one hand transmit and on the other hand reflect or absorb light to produce a modulated light beam, i.e. to produce a pattern of bright and dark pixels, thus creating a desired image. Because almost all illumination sources used in projectors generate non-polarised light, which is light comprising at least two polarisation directions, this light has to be polarised in an optical system either before it reaches the LCD or LCOS device(s) or in the device itself. This may be done by only selecting that part of the light which has a desired polarisation direction, e.g. using a polarising filter, which method, however, leads to a loss of light output for the projector.
To overcome this problem, typical LCD or LCOS projectors use a polarisation recuperation system that splits the unpolarised light from the lamp into two light beams which each have one of the two independent polarisation directions. Each light beam with a different polarisation direction follows a different optical path. The two independently polarised states for example can be two orthogonal linear polarisations or as another example a left and right circular polarisation. The light with one of the two polarisation directions is then converted into the other polarisation state by a retarder or a polarisation rotator. Thereafter, the light is cast on the LCD or LCOS devices via another optical path. This system avoids throwing away the light having the unwanted polarisation state and therefore almost doubles the efficiency of the illumination optics. A typical polarisation recuperation device comprises a polarising beam splitter or polarising beam splitter array and a number of polarisation conversion components, as shown for the device 100 in FIG. 1.
The light, generated by light sources 102, is directed to a polarisation recuperation device 104 using optical elements 106, 108. The light impinges on a number of entrance surfaces 110 of the polarising beam splitter array. In the array of polarising beam splitters 112 (PBS) this light is split-up into two different light paths. The two light paths are generally constructed in such a way that at the output surface 114, 116 of the PBS 112 they are put adjacent to each other. The output surface 114, 116 of the polarising beam splitters 112 consists out of first types of sub-surfaces 114 and second types of sub-surfaces 116, each emitting light with different polarisation state. One or more polarisation conversion components 118, i.e. for example a halfwave retarder, are then put on e.g. all second types of output sub-surfaces 116 emitting one of the two types of polarisation state. These polarisation conversion components 118 substantially change the polarisation state of the light coming out of these second type of output sub-surfaces 116 into the polarisation state of the light coming out of the first type of output sub-surfaces 114 that do not have a polarisation conversion component 118. Typically in such a polarisation recuperation device 104 the number of the output sub-surfaces 114, 116 is doubled with respect to the number of entrance surfaces 108. Corresponding therewith, typically the total surface area of the output sub-surfaces 114, 116 is doubled with respect to the total surface area of the input surfaces 108.